IMS analyzer and IMS analysis method

Abstract

To provide an IMS analysis device which can quickly determine and set the optimal drift voltage.SOLUTION: An IMS analysis device according to the present invention includes an electron emission element, a collector, an electrostatic gate electrode, an electrode for electric field formation, and a control unit that has a storage part. The control unit and the collector are provided to measure the current waveform of the current that flows when ions reach the collector. The control unit and the electron emission element are provided to feedback-control the voltage applied between a lower electrode and a surface electrode by setting a target value of the total area of the peak appearing in the current waveform. The storage part stores the relationship between the target value of the total area of the peak and the drift voltage. The control unit is provided so as to determine and set the drift voltage from the target value of the total area of the peak so that the half value width resolution obtained by dividing the arrival time of the peak by the half value width of the peak becomes high by using the relationship.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an IMS analyzer and an IMS analyzer. [Background technology]

[0002] Conventional IMS analyzers ionize sample gases using ion sources such as radiation and corona discharge. Because radiation and corona discharge have high energy, the sample gas may chemically decompose during ionization. In this case, many ions generated by the decomposition of the sample gas are detected by the detector, resulting in numerous peaks (detection peaks) in the IMS spectrum. This makes it difficult to identify the sample gas in IMS analysis.

[0003] IMS analyzers are known that use low-energy electrons emitted from an electron-emitting element to ionize a sample gas (see, for example, Patent Document 1). This analyzer can suppress the chemical decomposition of the sample gas, making it easier to identify the sample gas. When an electron emission element is used as an ion source, electrons emitted from the element first ionize molecules in the air, generating primary ions. When the sample gas to be measured is mixed with these primary ions, the target molecules in the sample gas are ionized by ionic molecular reactions, generating secondary ions. The primary and secondary ions are instantaneously injected into the drift region by the electrostatic gate electrode, move through the drift region due to the electric field formed by the drift voltage, and finally reach the collector, where the ion charge is transferred. By measuring the current generated by this charge transfer, the recovered current waveform (IMS spectrum, spectral waveform of the detected current against time) can be obtained. The time it takes for different ion species to move through the drift region (peak position, arrival time) differs. Therefore, the peaks corresponding to the ion species that appear in the IMS spectrum are normal distribution-like peak waveforms with specific peak positions (arrival times) and variances, and the IMS spectrum appears with superimposed peaks corresponding to the ion species.

[0004] On the other hand, it is known that the peak arrival time and full width at half maximum of the IMS spectrum change depending on the drift tube configuration (drift tube length, drift voltage) and the environment of the drift gas (temperature, pressure), and the following model equation (1) has been proposed to explain this phenomenon (see Non-Patent Literature 1).

[0005]

number

[0006] Peak full width at half maximum (FWHM) and FWHM resolution are commonly used as indicators to explain the resolution between adjacent peaks appearing in an IMS spectrum. FWHM resolution is the value obtained by dividing the peak arrival time in the IMS spectrum by its FWHM. The higher the FWHM resolution, the better the resolution between adjacent peaks, making it easier to identify individual components. IMS is an abbreviation for Ion Mobility Spectrometry. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2019-186190 [Non-patent literature]

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, in a conventional IMS analyzer, an IMS spectrum in which the peak of primary ions generated from air overlaps with the peak of secondary ions generated from the component to be detected may be measured. In this case, the peak position (arrival time) of the secondary ions cannot be clarified, and it is difficult to identify the component to be detected. Although it is sometimes possible to adjust the drift time and the full-width at half-maximum resolution and separate the overlapping peaks by setting the drift voltage to various values and performing preliminary analysis many times, a lot of preliminary analysis is required to find the optimal conditions. Also, the full-width at half-maximum resolution calculated using the above model formula (1) deviates from the resolution calculated from the measurement results. The present invention has been made in view of such circumstances, and provides an IMS analyzer capable of quickly determining and setting an optimal drift voltage.

Means for Solving the Problems

[0010] The present invention comprises an electron emission element, a collector, an electrostatic gate electrode, an electric field forming electrode, and a control unit having a memory unit, wherein the electron emission element has a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode, and the control unit and the electron emission element are configured to generate ions directly or indirectly by emitting electrons from the electron emission element by applying a voltage between the lower electrode and the surface electrode, and the control unit and the electric field forming electrode are configured to use the electric field forming electrode to form a potential gradient in the drift region between the electrostatic gate electrode and the collector such that the ions directly or indirectly generated by the electrons emitted from the electron emission element move to the collector The present invention provides an IMS analyzer, characterized in that it is provided to apply a drift voltage to a region, the control unit and the collector are provided to measure the current waveform of the current that flows when ions reach the collector, the control unit and the electron emission element are provided to set a target value for the total area of ​​the peaks appearing in the current waveform and to feedback control the voltage applied between the lower electrode and the surface electrode, the storage unit stores the relationship between the target value for the total area of ​​the peaks and the drift voltage, and the control unit is provided to use the relationship to determine and set the drift voltage from the target value for the total area of ​​the peaks so that the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak is high. [Effects of the Invention]

[0011] According to the present invention, the drift voltage can be determined and set to have high half-width resolution from a target value of the total area of ​​peaks appearing in the IMS spectrum (the current waveform) using the relationships stored in the memory unit. Therefore, the optimal drift voltage can be quickly determined and set. As a result, preliminary analysis to find the optimal drift voltage can be omitted, and IMS analysis can be performed quickly. Furthermore, even when the target value of the total peak area is changed for sensitivity adjustment, the optimal drift voltage can be quickly determined and set. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic cross-sectional view of an IMS analyzer according to one embodiment of the present invention. [Figure 2] This graph shows the change in the theoretical value of the full width at half maximum (FWHM) calculated using equation (1) with respect to the drift voltage, and the FWHM value calculated from the measurement results in the actual device. [Figure 3] This graph shows the change in the theoretical value of the full width at half maximum (FWHM) resolution with respect to the drift voltage, calculated using equation (1), and the FWHM resolution value calculated from the measurement results of the actual device. [Figure 4] This graph shows the change in the drift voltage of the half-width value calculated from measurement results on an actual device. [Figure 5] This graph shows the change in the value of the half-width resolution, calculated from measurement results on an actual device, with respect to the drift voltage. [Figure 6] This graph shows the change in the theoretical value of the full width at half maximum (FWHM) calculated using equation (2) with respect to the drift voltage, and the FWHM value calculated from the measurement results in the actual device. [Figure 7] This graph shows the change in the theoretical value of the full width at half maximum (FWHM) resolution calculated using equation (2) with respect to the drift voltage, and the FWHM resolution value calculated from the measurement results of the actual device. [Modes for carrying out the invention]

[0013] The IMS analyzer of the present invention comprises an electron emission element, a collector, an electrostatic gate electrode, an electric field forming electrode, and a control unit having a memory unit, wherein the electron emission element has a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode, and the control unit and the electron emission element are configured to generate ions directly or indirectly by emitting electrons from the electron emission element by applying a voltage between the lower electrode and the surface electrode, and the control unit and the electric field forming electrode are configured to form a potential gradient in the drift region between the electrostatic gate electrode and the collector such that the ions directly or indirectly generated by the electrons emitted from the electron emission element move to the collector, the electric field forming electrode The device is configured to apply a drift voltage to the drift region using electrodes, the control unit and the collector are configured to measure the current waveform of the current that flows when ions reach the collector, the control unit and the electron emission element are configured to set a target value for the total area of ​​the peaks appearing in the current waveform and to feedback control the voltage applied between the lower electrode and the surface electrode, the storage unit stores the relationship between the target value for the total area of ​​the peaks and the drift voltage, and the control unit is configured to use the relationship to determine and set the drift voltage from the target value for the total area of ​​the peaks so that the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak is high. The aforementioned relationship is dependent on the instrument configuration of the IMS analyzer and may be a relationship calculated based on the measurement results of a preliminary IMS analysis performed using the aforementioned IMS analyzer or an IMS analyzer having the same instrument configuration, or it may be a relationship that includes parameter values ​​calculated based on the measurement results of a preliminary IMS analysis. Such a relationship can be used universally for the same model of IMS analyzer.

[0014] Preferably, the memory unit stores the relationship between the target value of the total area of ​​the peak and the drift voltage, which is calculated based on the measurement results of preliminary IMS analysis, such that the half-width resolution is high, obtained by dividing the peak arrival time by the half-width of the peak. Preferably, the relationship described above is a relationship calculated based on measurement results obtained by repeatedly performing IMS analysis while changing the drift voltage while feedback-controlling the voltage applied between the lower electrode and the surface electrode, setting a target value for the total area of ​​the peaks, and performing this IMS analysis for each of the multiple target values.

[0015] Preferably, the storage unit stores the relationship as data or a relational expression. Preferably, the memory unit stores data corresponding to equations (2) and (3) or their relationships, as described later, as well as parameters a and b calculated based on the measurement results of preliminary experiments, or relationships derived by substituting parameters a and b into equations (2) and (3). This eliminates the need to conduct preliminary experiments with all possible combinations of the set domain, allowing experiments to be completed with only the minimum number of representative points. Furthermore, elements such as temperature and atmospheric pressure can also be reflected in the calculation results. The present invention also provides an IMS analysis method for feedback-controlling the voltage applied between the lower electrode and the surface electrode of an electron emission element by setting a target value for the total area of ​​peaks appearing in the IMS spectrum, and an IMS analysis method that includes the step of determining and setting the drift voltage from the target value for the total area of ​​peaks so as to increase the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak, using the relationship between the target value for the total area of ​​peaks appearing in the IMS spectrum and the drift voltage applied to the drift region between the electrostatic gate electrode and the collector.

[0016] One embodiment of the present invention will be described below with reference to the drawings. The configurations shown in the drawings and the following description are illustrative, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0017] Figure 1 is a schematic cross-sectional view of the IMS analyzer according to this embodiment. The IMS analyzer 40 of this embodiment comprises an electron emission element 2, a collector 6, an electrostatic gate electrode 8, electric field forming electrodes 9a to 9h, and a control unit 12 having a memory unit 13. The electron emission element 2 has a lower electrode 3, a surface electrode 4, and an intermediate layer 5 disposed between the lower electrode 3 and the surface electrode 4. The control unit 12 and the electron emission element 2 are configured to generate ions directly or indirectly by emitting electrons from the electron emission element 2 by applying a voltage between the lower electrode 3 and the surface electrode 4. The control unit 12 and the electric field forming electrodes 9a to 9h are configured to form a potential gradient in the drift region 11 between the electrostatic gate electrode 8 and the collector 6 such that the ions directly or indirectly generated by the electrons emitted from the electron emission element 2 move to the collector 6. The device is configured to apply a drift voltage to the drift region 11 using field-forming electrodes 9a to 9h, the control unit 12 and collector 6 are configured to measure the current waveform of the current that flows when ions reach the collector 6, the control unit 12 and electron emission element 2 are configured to set a target value for the total area of ​​the peaks appearing in the current waveform and to feedback control the voltage applied between the lower electrode 3 and the surface electrode 4, the storage unit 13 stores the relationship between the target value for the total area of ​​the peaks and the drift voltage, and the control unit 12 is configured to use this relationship to determine and set the drift voltage from the target value for the total area of ​​the peaks so that the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak is high.

[0018] The IMS analyzer 40 in this embodiment is a device that ionizes a sample gas and analyzes its mobility. The analyzer 40 may be a drift tube type IMS analyzer. The sample gas analyzed by the IMS analyzer 40 may be a gaseous sample or a liquid sample that has been vaporized.

[0019] The control unit 12 controls the IMS analyzer 40 and includes a memory unit 13. The control unit 12 may include, for example, a microcontroller having a CPU, memory, timer, input / output ports, etc. The memory unit 13 may be, for example, a hard disk, SSD, non-volatile memory, volatile memory, ROM, RAM, etc. The control unit 12 may also include an electric field control unit 26, a gate control unit 27, an element voltage control unit 17, a recovered current measurement unit 19, a power supply unit, etc. Furthermore, the control unit 12 may include wires for applying voltage, signal lines for sending and receiving control signals, wires for measuring recovered current, etc. The memory unit 13 stores the relationship between the target value of the total area of ​​peaks appearing in the IMS spectrum and the drift voltage. This will be explained later.

[0020] The electron emission element 2 is an electron-emitting element used to ionize components contained in air, sample gas, or carrier gas.

[0021] The IMS analyzer 40 of this embodiment has an analysis chamber 30 for analyzing target components contained in a sample gas. The analysis chamber 30 has an ionization region 10 for ionizing target components contained in the sample gas and generating ions (negative or positive ions) between the electron emission element 2 and the collector 6, and a drift region 11 (ion transfer region) for moving and separating the ions. An electrostatic gate electrode 8 is provided between the ionization region 10 and the drift region 11. The electron emission element 2 is positioned on the opposite side of the ionization region 10 from the electrostatic gate electrode 8, such that the surface electrode 4 faces the ionization region. The collector 6 is positioned on the opposite side of the drift region 11 from the electrostatic gate electrode. The electron emission element 2, collector 6, electric field forming electrodes 9a to 9h, electrostatic gate electrode 8, etc., are housed in the analysis chamber 30.

[0022] The sample gas injection unit 16 is the part that injects sample gas or carrier gas into the analysis chamber 30. The sample gas injection unit 16 may also be configured to inject a mixed gas of sample gas and carrier gas, or a carrier gas without the sample gas, into the analysis chamber 30. For example, in preliminary analysis before measuring the sample gas (e.g., analysis to determine analytical conditions or analysis to calculate databases and relational formulas used to determine the optimal drift voltage), the sample gas injection unit 16 can inject a carrier gas without the sample gas into the analysis chamber 30. Furthermore, when measuring the sample gas, the sample gas injection unit 16 can inject a mixed gas of sample gas and carrier gas into the analysis chamber 30. The flow rate of the gas supplied to the analysis chamber 30 by the sample gas injection unit 16 is, for example, 200 mL / min. The target components contained in the sample gas are analyzed by ion mobility analysis. When the sample is a gas, the sample gas injection unit 16 can be configured to continuously supply the sample gas to the analysis chamber 30. When the sample is a liquid, the sample gas injection unit 16 can have a vaporization chamber, and the sample gas vaporized in this vaporization chamber can be injected into the analysis chamber 30. The carrier gas is a gas injected into the analysis chamber 30 along with the sample gas, and is, for example, air containing moisture. The components contained in the carrier gas are ionized by electrons emitted from the electron emission element 2 to generate primary ions.

[0023] The drift gas injection unit 15 is a part provided for injecting drift gas into the analysis chamber 30. The drift gas is a gas that flows in the opposite direction to the direction of ion movement (from the electrostatic gate electrode 8 to the collector 6) in the drift region 11, and acts as resistance when ions move through the drift region 11. The drift gas may be purified air from the atmosphere (clean air), air supplied from a compressed air cylinder, or purified air discharged from the analysis chamber 30 by the exhaust unit 20. The flow rate of the drift gas supplied to the analysis chamber 30 by the drift gas injection unit 15 can be greater than the flow rate of the gas supplied to the analysis chamber 30 by the sample gas injection unit 16. The flow rate of the drift gas is, for example, 500 mL / min.

[0024] The exhaust section 20 is a part provided to discharge gas from the analysis chamber 30. The exhaust section 20 is provided to discharge drift gas and sample gas from the analysis chamber 30. The exhaust section 20 may be provided to forcibly exhaust the gas from the analysis chamber 30 using an exhaust fan or the like, or it may be provided to allow the gas from the analysis chamber 30 to be naturally exhausted.

[0025] The sample gas injection section 16 and the exhaust section 20 can be configured so that the sample gas flows through the ionization region 10. This allows the components contained in the sample gas to be ionized directly or indirectly by electrons emitted from the surface electrode 4 of the electron emission element 2 in the ionization region 10, thereby generating negative or positive ions.

[0026] The drift gas injection section 15 and the exhaust section 20 are provided so that the drift gas flows from the collector side to the electrostatic gate electrode side in the drift region 11. For example, the drift gas injection section 15 can be provided to supply drift gas to the drift region 11 from the collector side, and the exhaust section 20 can be provided to exhaust the drift gas from the opening (gas outlet) of the housing 28 around the ionization region 10.

[0027] The electron emission element 2 is an element provided to emit electrons from the surface electrode 4, and is an element that directly or indirectly ionizes the target component contained in the sample gas with these emitted electrons to generate negative or positive ions. The electron emission element 2 has a lower electrode 3, a surface electrode 4, and an intermediate layer 5 disposed between the lower electrode 3 and the surface electrode 4.

[0028] The surface electrode 4 is an electrode located on the surface of the electron emission element 2. The surface electrode 4 preferably has a thickness of 10 nm to 100 nm. The material of the surface electrode 4 is, for example, gold or platinum. The surface electrode 4 may also be composed of multiple metal layers. Even if the surface electrode 4 has a thickness of 40 nm or more, it may have multiple openings, gaps, and thinned portions with a thickness of 10 nm or less. Electrons flowing through the intermediate layer 5 can pass through or transmit through these openings, gaps, and thinned portions, and electrons can be emitted from the surface electrode 4. Such openings, gaps, and thinned portions can also be formed by applying an element voltage between the lower electrode 3 and the surface electrode 4.

[0029] The lower electrode 3 is an electrode that faces the surface electrode 4 via an intermediate layer 5. The lower electrode 3 may be a metal plate, or a metal layer or conductive layer formed on an insulating substrate or film. If the lower electrode 3 is made of a metal plate, this metal plate may be the substrate of the electron emission element 2. The material of the lower electrode 3 may be, for example, aluminum, stainless steel, or nickel. The thickness of the lower electrode 3 is, for example, 200 μm or more and 1 mm or less.

[0030] The intermediate layer 5 is a layer through which electrons flow due to the electric field formed by applying an element voltage between the surface electrode 4 and the lower electrode 3. The intermediate layer 5 may be semiconductive. The intermediate layer 5 may contain at least one of insulating resin, insulating fine particles, and metal oxide. Preferably, the intermediate layer 5 contains conductive fine particles. The thickness of the intermediate layer 5 can be, for example, 0.5 μm or more and 1.8 μm or less. The intermediate layer 5 is, for example, a silicone resin layer having silver fine particles dispersed in it.

[0031] The electron emission element 2 may have an insulating layer 29 between the surface electrode 4 and the lower electrode 3. This insulating layer 29 may have an opening. The opening in the insulating layer 29 is provided to define the electron emission region of the surface electrode 4. Since electrons cannot flow through the insulating layer 29, electrons flow to the intermediate layer 5 corresponding to the opening in the insulating layer 29 and are emitted from the surface electrode 4. Therefore, by providing an insulating layer 29 with an opening, the electron emission region formed on the surface electrode 4 is defined. The electron emission region can be, for example, a 5 mm square region and can be freely designed in accordance with the size of the openings of the electric field forming electrodes 9a to 9h and the collector 6.

[0032] The surface electrode 4 and the lower electrode 3 can each be electrically connected to the control unit 12 (element voltage control unit 17). The element voltage control unit 17 is provided to control the magnitude of the element voltage (driving voltage of the electron emission element 2) applied between the surface electrode 4 and the lower electrode 3. When the element voltage control unit 17 is used to make the potential of the lower electrode 3 substantially the same as the potential of the surface electrode 4 (setting the element voltage to 0V), no current flows through the intermediate layer 5 and no electrons are emitted from the electron emission element 2.

[0033] When an element voltage (driving voltage) is applied between the lower electrode 3 and the surface electrode 4 using the element voltage control unit 17 such that the potential of the lower electrode 3 is lower than the potential of the surface electrode 4, a current flows through the intermediate layer 5, and electrons that have flowed through the intermediate layer 5 pass through the surface electrode 4 and are emitted into the ionization region 10. The element voltage applied between the lower electrode 3 and the surface electrode 4 to emit electrons from the electron emission element 2 can be, for example, 5V to 40V, and preferably 10V to 18V. By adjusting the magnitude of the element voltage using the element voltage control unit 17, the current flowing through the intermediate layer 5 changes, and the amount of electrons emitted from the electron emission element 2 changes. Therefore, by increasing the element voltage, the amount of ions generated in the ionization region 10 can be increased, and the peak intensity of the peaks appearing in the IMS spectrum measured using the recovery current measurement unit 19 can be increased (detection sensitivity increases). However, increasing the element voltage increases the full width at half maximum of the peaks appearing in the IMS spectrum. This is thought to be because increasing the element voltage increases the amount of ions passing through the electrostatic gate electrode 8, thus increasing the interaction between ions. Also, when the full width at half maximum of the peaks appearing in the IMS spectrum increases, the possibility of multiple peaks overlapping increases. Furthermore, adjusting the magnitude of the element voltage also changes the energy of the electrons emitted from the electron emission element 2.

[0034] When IMS analysis is repeated, the control unit 12 may set a target value for the total area of ​​peaks appearing in the IMS spectrum and feedback control the element voltage applied between the lower electrode 3 and the surface electrode 4. This makes it possible to keep the total amount of ions generated in the ionization region 10 substantially constant. Furthermore, by changing the target value for the total area of ​​peaks, the total amount of ions generated in the ionization region 10 can be changed, thereby changing the sensitivity of the IMS analyzer.

[0035] When a carrier gas (air containing moisture) without sample gas is injected into the analysis chamber 30 using the sample gas injection unit 16, and drift gas (air) is injected into the analysis chamber 30 using the drift gas injection unit 15, when electrons are emitted from the electron emission element 2 into the ionization region 10, the electrons immediately collide with air components and form primary ions (negative or positive ions). When electrons emitted from the electron emission element 2 adhere to gaseous components near the surface electrode 4 (electron adhesion phenomenon), negative ions of the gaseous components are generated. If the energy of the electrons emitted from the electron emission element 2 is higher than the ionization energy of the gaseous components near the surface electrode 4, positive ions of the gaseous components are generated. These generated primary ions move toward the electrostatic gate electrode 8 due to the electric field formed in the analysis chamber 30. Primary ions are, for example, oxygen ions obtained by ionizing oxygen gas in the air. In this case, the ionization region 10 contains an amount of primary ions corresponding to the amount of electrons emitted by the electron emission element 2. The amount of primary ions can be adjusted by adjusting the element voltage applied between the surface electrode 4 and the lower electrode 3 (adjusting the amount of electrons emitted by the electron emission element 2).

[0036] When a mixed gas of sample gas and carrier gas (air containing moisture) is injected into the analysis chamber 30 using the sample gas injection unit 16, and drift gas (air) is injected into the analysis chamber 30 using the drift gas injection unit 15, when electrons are emitted from the electron emission element 2 into the ionization region 10, the electrons immediately collide with air components and generate primary ions (negative or positive ions). These primary ions move toward the electrostatic gate electrode 8 due to the electric field formed in the analysis chamber 30. Furthermore, these primary ions transfer charge to the target component contained in the sample gas in the ionization region 10, generating negative ions (secondary ions) or positive ions (secondary ions) of the target component contained in the sample gas (ionic molecular reaction). In other words, negative or positive ions of the target component contained in the sample gas can be indirectly generated in the ionization region 10 using the electron emission element 2. At this time, the ionization region 10 contains both ions (secondary ions) generated from the target component contained in the sample gas and primary ions.

[0037] The electric field forming section 7 is a part for forming a potential gradient in the region between the electron emission element 2 and the collector 6. The electric field forming section 7 is provided to form a potential gradient such that ions move from the electron emission element side to the collector side. When the IMS analyzer 40 detects negative ions (negative ion mode), the control unit 12 (electric field control unit 26) applies a voltage to the electric field forming section 7 so that a potential gradient is formed such that the potential on the electron emission element side is lower than the potential on the collector side. When the IMS analyzer 40 detects positive ions (positive ion mode), the control unit 12 (electric field control unit 26) applies a voltage to the electric field forming section 7 so that a potential gradient is formed such that the potential on the electron emission element side is higher than the potential on the collector side.

[0038] The electric field forming section 7 may include a plurality of electric field forming electrodes 9a to 9h. The shape of the electric field forming electrodes 9a to 9h is not limited as long as they can form a potential gradient in the region between the electron emission element 2 and the collector 6, but they may be, for example, ring-shaped electrodes or arch-shaped electrodes. The plurality of electric field forming electrodes 9a to 9h are arranged in a line such that an ionization region 10 and a drift region 11 (ion transfer region) are formed inside the ring or inside the arch. The electric field forming electrodes 9a to 9h constituting the electric field forming section 7 are electrically connected to the electric field control unit 26 of the control unit 12. In addition, the grid electrode 25, collector 6, and the surface electrode 4 or lower electrode 3 of the electron emission element 2 may function as the electric field forming section 7. The control unit 12 can form a potential gradient in the ionization region 10 and the drift region 11 by controlling the potentials of the electric field forming electrodes 9a to 9h, the surface electrode 4 of the electron emission element 2, etc. Of the voltages that the control unit 12 applies to the electric field forming electrodes 9a to 9h, etc., in order to form the potential gradient, the voltage applied to the drift region 11 is the drift voltage.

[0039] Two adjacent electric field forming electrodes 9a to 9h included in the electric field forming section 7 can be electrically connected with a resistor in between. This allows a potential difference to be generated between the two adjacent electric field forming electrodes 9a to 9h, and by generating this potential difference between each electrode, a uniform electric field can be formed in the region between the electron emission element 2 and the collector 6.

[0040] For example, in the analyzer 40 shown in Figure 1, the electric field forming unit 7 includes multiple electric field forming electrodes 9a to 9h, and two adjacent electric field forming electrodes 9a to 9h are electrically connected with a resistor in between. Furthermore, the electrode 9h closest to the collector 6 is electrically connected to the grid electrode 25 with a resistor in between. The grid electrode 25 is connected to ground, for example, with a resistor in between. The potential of the electrode 9a furthest from the collector 6 can be controlled by the control unit 12. For example, the control unit 12 can apply a drift voltage so that the potential of electrode 9a becomes, for example, -3000V. Also, since the grid electrode 25 is connected to ground with a resistor in between, its potential is near 0V. Furthermore, because two adjacent electric field forming electrodes 9a to 9h are electrically connected with a resistor in between, the potential of the multiple electric field forming electrodes 9a to 9h arranged in a line increases in a stepwise manner as they approach the collector 6. Therefore, a potential gradient can be formed in the region between the electron emission element 2 and the collector 6 (ionization region 10 and drift region 11) where the potential gradually increases as you approach the collector 6. However, this potential gradient changes near the electrostatic gate electrode 8 due to the potential of the electrostatic gate electrode 8. Furthermore, the control unit 12 can control the potential of the electron emission element 2, taking into account the potential gradient formed by the multiple electric field forming electrodes 9a to 9h. In addition, the electron emission element 2 functions as part of the electric field forming unit 7, and the multiple electric field forming electrodes 9a to 9h and the electron emission element 2 can work together to form a potential gradient.

[0041] The electrostatic gate electrode 8 is an electrode positioned between the ionization region 10 and the drift region 11, and controls the implantation of ions generated in the ionization region 10 into the drift region 11 by utilizing the electrostatic interaction between the ions and the electrostatic gate electrode 8. The electrostatic gate electrode 8 is, for example, a grid-shaped electrode (shutter grid). The electrostatic gate electrode 8 can be arranged in a line with the multiple electric field forming electrodes 9a to 9h that constitute the electric field forming section 7. The electrostatic gate electrode 8 can be electrically connected to the gate control section 27 of the control section 12. Furthermore, the electrostatic gate electrode 8 is provided in such a way that it can change the potential gradient formed by the electric field forming section 7.

[0042] The gate control unit 27 changes the potential of the electrostatic gate electrode 8 so as to instantaneously change it from a low-potential closed state (where the potential of the electrostatic gate electrode 8 is low, preventing ions from the ionization region 10 from passing through the electrostatic gate electrode 8 and moving to the drift region 11) to a high-potential closed state (where the potential of the electrostatic gate electrode 8 is high, preventing ions from the ionization region 10 from passing through the electrostatic gate electrode 8 and moving to the drift region 11), or so as to instantaneously change it from a high-potential closed state to a low-potential closed state. This allows the electrostatic gate electrode 8 to be open for a very short time, and ions from the ionization region 10 to be injected into the drift region 11 for only this short time. Therefore, ions from the ionization region 10 can be injected into the drift region 11 in a single-pulse manner.

[0043] Negative or positive ions injected into the drift region 11 move toward the collector 6 due to the potential gradient formed by the electric field generating unit 7, and reach the collector 6. At this time, the negative or positive ions move against the flow of drift gas. This flow of drift gas acts as resistance for the negative or positive ions moving from the electrostatic gate electrode 8 toward the collector 6. The magnitude of this resistance (ion mobility) differs depending on the ion species. Generally, mobility is inversely proportional to the collision cross-section of the ion (ion size), so the larger the collision cross-section of the ion, the longer it takes for the ion to reach the collector 6 (larger ions collide with air molecules in the drift gas more frequently, slowing down their movement speed and delaying the time it takes to reach the collector 6). Therefore, the time from when the ions are injected into the drift region 11 by the electrostatic gate electrode 8 until they reach the collector 6 (arrival time, peak position) differs depending on the ion species (negative or positive ions). Therefore, based on this arrival time (peak position), it becomes possible to identify negative ions or positive ions (target components contained in the sample). In addition, multiple target component ions contained in the sample gas can be separated in the drift region 11.

[0044] The collector 6 is a metallic component that collects the charge of negative or positive ions. The collector 6 can be electrically connected to the recovery current measuring unit 19 of the control unit 12. The recovery current measuring unit 19 is provided to measure the recovery current generated when negative or positive ions transfer charge to the collector 6 in a time series. This makes it possible to measure the current waveform (IMS spectrum) of the recovery current.

[0045] Because different ion species have different travel times (peak positions, arrival times) through the drift region, multiple ions injected into the drift region 11 in single pulses using the electrostatic gate electrode 8 are separated into various ions as they travel through the drift region 11, and these ions arrive at the collector 6 with a time lag. Furthermore, even ions of the same species diffuse to some extent as they travel through the drift region 11. For this reason, the peaks corresponding to the ion species that appear in the IMS spectrum are normal distribution-like peak waveforms with specific peak positions (arrival times) and variances, and each peak corresponding to the ion species appears in the IMS spectrum. It is possible to calculate the mobility from these peak positions (arrival times) and identify the ion components. In addition, since the peak height or peak area of ​​the current waveform of the recovered current corresponds to the amount of charge transferred by the various ions to the collector 6, it becomes possible to quantitatively analyze the target component based on the peak height or peak area.

[0046] When the ion mobilities of multiple types of ions are similar in the drift region 11, multiple peaks may overlap and appear in the IMS spectrum. In such cases, it becomes difficult to determine the peak position (arrival time) and peak area of ​​the peaks appearing in the IMS spectrum, making it difficult to distinguish the ion components. In particular, since a large peak of primary ions generated from air appears in the IMS spectrum, if the ion mobility of the target component ion in the drift region 11 is similar to that of the primary ion, it becomes difficult to determine the peak position (arrival time) and peak area of ​​this target component ion.

[0047] The magnitude of the peak of the target component's ions (secondary ions) depends on the sample gas concentration and ease of ionization. At low sample gas concentrations, the peak may be too small to be discernible. In such cases, increasing the amount of primary ions, which are the charge source for the secondary ions, can increase the amount of secondary ions, making the peak waveform visible. However, increasing the amount of primary ions increases the primary ion peak and the width of the peak distribution, which is a factor reducing the resolution between adjacent peaks. As indices for explaining this resolution, the full width at half maximum (FWHM) of the peak and the FWHM resolution (also referred to as resolution), which is the arrival time divided by the FWHM, are often used. The higher the FWHM resolution, the higher the separation performance between adjacent peaks and the easier it is to identify individual components.

[0048] On the other hand, it is known that the arrival time and the FWHM change depending on the configuration of the drift tube (drift tube length, drift voltage) and the environment of the drift gas (temperature, pressure), and a model formula (1) for explaining its characteristics has been proposed (see Non-Patent Document 1).

[0049]

Equation

[0050] Among these parameters, the drift voltage has high controllability as a device and a great influence on the FWHM and the FWHM resolution. FIGS. 2 and 3 are graphs showing the relationship curves of the FWHM or resolution with respect to the drift voltage calculated by substituting the following values of the Boltzmann constant, elementary charge, drift length, absolute temperature, and ion mobility into formula (1), and the measurement results in an actual device. The fixed parameters used in the calculation are shown below. Boltzmann constant K B = 1.381×10 -23 J·K -1 Elementary charge e=1.602×10 -19 C Drift length L = 8.0 cm Absolute temperature T = 298K Initial half-width W inj = 0.05ms Ion mobility K = 2.2 cm 2 ·V -1 ·s -1

[0051] The curve in Figure 3, which shows the relationship between drift voltage and resolution, reveals that the resolution has a peak at a specific drift voltage. However, while the trend in actual measurements with the real device is the same as that of the relationship calculated from the model equation, the absolute values ​​are plotted differently. One possible reason for this is that the magnitude of the ion amount, which is not included in equation (1), is influencing the results. Here, the total area (or its target value) of the peaks appearing in the measured waveform (IMS spectrum) is used as an alternative indicator for controlling the amount of primary ions generated. Since the sample gas components receive charge from primary ions to become secondary ions, the total amount of ions (charge amount) of primary and secondary ions combined can be used as an indicator of the amount of primary ions initially generated. Furthermore, since these ion amounts are ultimately measured as peaks appearing in the recovered current waveform (IMS spectrum), the total area (cumulative value) of these peaks can be used as an indicator of the initial amount of primary ions generated.

[0052] On the other hand, increasing the voltage applied to the electron emission element 2 (the voltage applied between the lower electrode 3 and the surface electrode 4) increases the amount of primary ions generated, while decreasing the applied voltage decreases the amount of primary ions generated. Therefore, by setting a target value for the total area of ​​the peaks appearing in the IMS spectrum and repeating the IMS analysis while feedback-controlling the voltage applied between the lower electrode 3 and the surface electrode 4, the amount of primary ions generated can be controlled to any desired level. The target value for the total peak area corresponds to the sensitivity of the IMS analysis; increasing the target value for the total peak area increases the sensitivity of the IMS analysis.

[0053] Using the actual apparatus, the target value S for the total peak area was increased in increments of 100 pA·ms from 100 pA·ms to 5000 pA·ms, and the drift voltage was varied within the range of 500 V to 6500 V while feedback-controlling the voltage applied between the lower electrode 3 and the surface electrode 4, and the IMS analysis was repeated. Figure 4 is a graph showing the change in half-width calculated from the measurement results (IMS spectrum) when the IMS analysis was repeated with the target value S set to 100 pA·ms, 1500 pA·ms, or 4000 pA·ms and the drift voltage was varied. Figure 5 is a graph showing the change in resolution calculated from the measurement results (IMS spectrum). Table 1 is a table showing the relationship between the target value S and the drift voltage, calculated based on the measurement results to maximize the resolution. Table 1 omits the target values ​​400pA·ms to 1300pA·ms, 1700pA·ms to 3800pA·ms, 4200pA·ms to 4700pA·ms, and the drift voltages corresponding to these target values ​​S.

[0054] As shown in the graph in Figure 5, the resolution changes in a convex curve, and it was found that the resolution is maximized at different drift voltages depending on the target value S of the total peak area. For example, when the target value S is set to 100 pA·ms, the resolution is maximized when the drift voltage is set to approximately 3500 V; when the target value S is set to 1500 pA·ms, the resolution is maximized when the drift voltage is set to approximately 4200 V; and when the target value S is set to 4000 pA·ms, the resolution is maximized when the drift voltage is set to approximately 5000 V. Thus, it was found that changing the target value S of the total peak area changes the drift voltage at which the resolution is maximized. Table 1 shows this maximum drift voltage for each target value S (target total area).

[0055] [Table 1]

[0056] The memory unit 13 stores the relationship between the target value S of the total peak area and the drift voltage, which is calculated based on the measurement results to increase the half-width resolution, as shown in Table 1. The memory unit 13 may store such a relationship as data (e.g., a database or table), or it may store it as a linear regression equation or a curved regression equation calculated from the data. These data can be obtained by setting a target value S for the total peak area, repeatedly performing IMS analysis while changing the drift voltage while feedback-controlling the voltage applied between the lower electrode 3 and the surface electrode 4, and performing this IMS analysis for each of the multiple target values ​​S. It is sufficient to acquire one set of data for each device set.

[0057] When actually measuring a sample gas, first, the desired target value S (sensitivity) to be set for the measurement is determined, the drift voltage is determined using the data stored in the memory unit 13 or the regression equation, and by setting the target value S and drift voltage and performing IMS analysis, it becomes possible to measure under conditions with the best possible resolution at the determined target value S (corresponding to the primary ion generation amount). Furthermore, if the target value S is changed to adjust the sensitivity, the drift voltage can be determined and reset using the same procedure, making it possible to continue measurements with optimal resolution at all times. Furthermore, when performing IMS analysis by setting the target value S to the value between two adjacent target values ​​S contained in the data stored in the memory unit 13, nearly optimal control can be achieved by performing interpolation calculations based on the preceding and succeeding data.

[0058] If the relationship between the target value S of the total peak area and the drift voltage (data, linear regression equation, or curved regression equation) stored in the memory unit 13 is calculated based on a large number of data points, the drift voltage can be set to the optimal value with high accuracy, and IMS analysis can be performed with optimal resolution. However, this will also increase the number of preliminary experiments required to acquire the data needed to calculate the aforementioned relationship. In addition to drift voltage, other parameters that affect the full width at half maximum (FMAX) and resolution include the temperature and pressure of the drift gas. While the impact of variations in these parameters in a realistic measurement environment is small, being able to reflect these values ​​would allow for more precise adjustment of the drift voltage. However, conducting experiments on various conditions combining ion generation (target value S for total peak area), drift voltage, temperature, and pressure to calculate the relationships necessary to determine the optimal drift voltage would require an enormous amount of experimentation, making it impractical. Therefore, the inventors of this application propose original model equations (2) and (3) that incorporate the influence of ion generation amount (target value S of total peak area) into model equation (1), and propose a method that enables the determination and setting of the optimal drift voltage on a mathematical basis.

[0059]

number

[0060] α = a × S + b ·····(3) In the equation, α represents the diffusion term correction coefficient, S represents the target value for the total area of ​​peaks appearing in the IMS spectrum, a represents the slope of the approximation formula, and b represents the intercept of the approximation formula.

[0061] The denominator of equation (1) (full width at half maximum) is the full width at half maximum W immediately after passing through the electrostatic gate electrode. inj And the subsequent increment in the full width at half maximum (diffusion term) W due to diffusion within the drift region. diffIt is represented as follows. In equation (2), by multiplying the diffusion term by a correction coefficient α based on the target value of the total peak area, a model equation with high reproducibility for measured data can be obtained. In this example, as shown in equation (3), α is a linear equation with respect to the target value S of the total peak area, and the slope a and intercept b of the approximation equation are determined based on the measured data. There are no particular restrictions on the order of the approximation equation, and it is possible to use a higher-order equation, but the simplest linear equation can ensure sufficient reproducibility for practical use. The approximate parameters a and b can be determined from experimental data, for example, using the least squares method. By rearranging equations (2) and (3) with respect to the parameters a and b to be identified, the relationships shown in equations (4) to (6) can be derived.

[0062] W 0.5 2 =W inj 2 +W diff ×(aS+b)····(4) W 0.5 2 =W inj 2 +a×W diff ×S+b×W diff ...(5) Y = c + aX1 + bX2 ····(6) Y=W 0.5 2 ...(6-1) c=W inj 2 ...(6-2) X1=W diff ×S····(6-3) X² = W diff ...(6-4)

[0063] Since Y, X1, and X2 in equation (6) are determined from the specified experimental conditions and measurement results, it is possible to identify the parameters a, b, and c by least squares or other methods by collecting data under multiple conditions. Table 2 shows the conditions and measurement results of the preliminary experiment for parameter identification, as well as examples of identified parameters. St represents the target value for the total peak area, and Sm represents the measured value for the total peak area.

[0064] [Table 2]

[0065] Figure 6 shows a graph illustrating the relationship between drift voltage and full width at half maximum (FWHM) (solid, dotted, dashed lines) (estimated curve) derived by substituting the identified parameters (a, b, c) and target peak area S' of 250 pA·ms, 1000 pA·ms, or 2000 pA·ms into equations (2) and (3), respectively, and the FWHM (circle, triangle, square) for drift voltage calculated from experimental results of IMS analysis performed with a target peak area St of 250 pA·ms, 1000 pA·ms, or 2000 pA·ms.

[0066] Figure 7 shows a graph illustrating the relationship between drift voltage and half-width resolution (solid, dotted, dashed lines) (estimated curve) derived by substituting the identified parameters (a, b, c) and target peak area S' of 250 pA·ms, 1000 pA·ms, or 2000 pA·ms into equations (2) and (3), and the half-width resolution (circle, triangle, square) with respect to drift voltage calculated from experimental results of IMS analysis performed with a target peak area St of 250 pA·ms, 1000 pA·ms, or 2000 pA·ms.

[0067] As can be seen from the graphs in Figures 6 and 7, the full width at half maximum (FWHM) or FWHM resolution calculated from the relationship between drift voltage and FWHM resolution, derived by substituting the parameters (a, b, c, α) identified from the measurement results of the preliminary experiment into equations (2) and (3), was found to agree with the FWHM or FWHM resolution calculated from the experimental results of the IMS analysis with high accuracy.

[0068] The memory unit 13 of the control unit 12 can store relational equations derived by substituting parameters (a, b, c, α) identified from the measurement results of preliminary experiments into equations (2) and (3). Furthermore, parameter c may be a fixed value rather than one identified based on experimental results. By substituting the target value S for the total peak area used in IMS analysis into these relational equations, a relational equation for the full width at half maximum (FWHM) with respect to the drift voltage can be introduced. The drift voltage at the point where the FWHM resolution is maximized in the curve representing this relational equation can then be set as the drift voltage used in IMS analysis. This enables measurement under optimal resolution conditions at the determined target value S (corresponding to the primary ion generation amount). Note that the memory unit 13 may store the parameters and the relational equations separately. Additionally, temperature may be substituted into the relational equation along with the target value S. [Explanation of symbols]

[0069] 2: Electron emission element 3: Lower electrode 4: Surface electrode 5: Intermediate layer 6: Collector 7: Electric field forming section 8: Electrostatic gate electrode 9a~9h: Electrodes for electric field formation 10: Ionization region 11: Drift region 12: Control section 13: Memory section 15: Drift gas injection section 16: Sample gas injection section 17: Element voltage control section 19: Recovery current measurement section 20: Exhaust section 22: Element holder 25: Grid electrode 26: Electric field control section 27: Gate control section 28: Housing 29: Insulating layer 30: Analysis chamber 40: IMS analyzer

Claims

1. It comprises an electron emission element, a collector, an electrostatic gate electrode, an electric field forming electrode, and a control unit having a memory unit. The electron emission element comprises a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode. The control unit and the electron emission element are provided to generate ions directly or indirectly by emitting electrons from the electron emission element by applying a voltage between the lower electrode and the surface electrode, The control unit and the electric field forming electrode are provided such that a drift voltage is applied to the drift region using the electric field forming electrode so as to form a potential gradient in the drift region between the electrostatic gate electrode and the collector that moves ions directly or indirectly generated by electrons emitted from the electron emission element to the collector, The control unit and the collector are provided to measure the current waveform of the current that flows when ions reach the collector. The control unit and the electron emission element are provided to set a target value for the total area of ​​the peaks appearing in the current waveform and to provide feedback control of the voltage applied between the lower electrode and the surface electrode. The memory unit stores the relationship between the target value of the total area of ​​the peaks and the drift voltage. The IMS analyzer is characterized in that the control unit is provided to determine and set the drift voltage from a target value of the total area of ​​the peak so as to increase the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak using the relationship described above.

2. The IMS analyzer according to claim 1, wherein the memory unit stores as the relationship between a target value for the total area of ​​the peak and the drift voltage, calculated based on the measurement results of a preliminary IMS analysis, such that the half-width resolution is increased by dividing the peak arrival time by the half-width of the peak.

3. The IMS analyzer according to claim 1, wherein the relationship is calculated based on measurement results obtained by repeatedly performing IMS analysis by changing the drift voltage while feedback-controlling the voltage applied between the lower electrode and the surface electrode, setting a target value for the total area of ​​the peaks, and performing this IMS analysis for each of the multiple target values.

4. The IMS analyzer according to any one of claims 1 to 3, wherein the storage unit stores the relationship as data or a relational expression.

5. The IMS analyzer according to claim 1, wherein the memory unit stores data corresponding to equations (2) and (3) or the relationship between them, as well as parameters a and b calculated based on the measurement results of a preliminary experiment, or the relationship derived by substituting parameters a and b into equations (2) and (3). [Math 1] In the formula, R represents the half-width resolution, and t D represents the arrival time of the peak appearing in the IMS spectrum, W 0.5 represents the full width at half maximum of the aforementioned peak, W inj represents the initial half-width of the peak (half-width immediately after passing through the electrostatic gate electrode), L represents the drift length (length from the electrostatic gate electrode to the collector), and U D represents the drift voltage, T represents the absolute temperature, and K B represents the Boltzmann constant, e represents the elementary charge, K represents the ion mobility, and W represents the elementary charge. diff represents the diffusion term, and α represents the diffusion term correction coefficient. α=a×S+b・・・(3) In the equation, α represents the diffusion term correction coefficient, S represents the target value for the total area of ​​peaks appearing in the IMS spectrum, a represents the slope of the approximation formula, and b represents the intercept of the approximation formula.

6. An IMS analysis method that sets a target value for the total area of ​​peaks appearing in the IMS spectrum and feedback-controls the voltage applied between the lower electrode and the surface electrode of an electron emission element, An IMS analysis method comprising the step of determining and setting the drift voltage from the target value of the total area of ​​the peaks to be observed in the IMS spectrum, using the relationship between the target value of the total area of ​​the peaks appearing in the IMS spectrum and the drift voltage applied to the drift region between the electrostatic gate electrode and the collector, such that the half-width resolution obtained by dividing the peak arrival time by the half-width of the peak is high.

Images